TechFindings

So, you get this annoying error (in Debian, some Ubuntu, or perhaps any other Linux or even Unix system):

locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory

Perhaps you get it when running apt-get?
You Google for answers and you find nothing useful and/or just contradictory information?
You are connecting over SSH?
You are connecting from another system (like Mac OS X)?
Read on…
In it simplest incarnation, the problem looks like this.

$ man some-none-existing-program
man: can't set the locale; make sure $LC_* and $LANG are correct
No manual entry for some-none-existing-program

$ locale
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory
LANG=en_US.UTF-8
LANGUAGE=en_GB:en
LC_CTYPE=UTF-8
LC_NUMERIC=sv_SE.UTF-8
LC_TIME=sv_SE.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY=sv_SE.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=sv_SE.UTF-8
LC_NAME=sv_SE.UTF-8
LC_ADDRESS=sv_SE.UTF-8
LC_TELEPHONE=sv_SE.UTF-8
LC_MEASUREMENT=sv_SE.UTF-8
LC_IDENTIFICATION=sv_SE.UTF-8
LC_ALL=

What happens here, in the second case is that “UTF-8” is not a valid LC_CTYPE (in Debian) and since LC_ALL is not set it can not fall back properly. That is all. Why is LC_CTYPE invalid? Perhaps because you have used ssh from Mac OS X (or something else):

mac $ locale
LANG=
LC_COLLATE="C"
LC_CTYPE="UTF-8"
LC_MESSAGES="C"
LC_MONETARY="C"
LC_NUMERIC="C"
LC_TIME="C"
LC_ALL=

mac $ ssh user@debianhost

The point is, here LC_CTYPE=UTF-8, and it is valid in OS X. But it is not valid in Debian. That is all there is to it. Try:

# This will fix the problem
mac $ LC_CTYPE=en_US.UTF-8 ssh user@debianhost

# This will produce the problem even from a Debian machine
debianclient $ LC_CTYPE=UTF-8 ssh user@debianhost

# This will (kind of) eleminate the problem when already logged in
$ LC_CTYPE=en_US.UTF-8 locale
-bash: warning: setlocale: LC_CTYPE: cannot change locale (UTF-8)
LANG=en_US.UTF-8
LANGUAGE=en_GB:en
LC_CTYPE=en_US.UTF-8
LC_NUMERIC=sv_SE.UTF-8
LC_TIME=sv_SE.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY=sv_SE.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=sv_SE.UTF-8
LC_NAME=sv_SE.UTF-8
LC_ADDRESS=sv_SE.UTF-8
LC_TELEPHONE=sv_SE.UTF-8
LC_MEASUREMENT=sv_SE.UTF-8
LC_IDENTIFICATION=sv_SE.UTF-8
LC_ALL=

So, the problem is that all the guides on the internet presume you have a problem with your locales on your server, which you dont! The locale is probably fine. It is just that the ssh client computer has an LC_CTYPE which does not happen to be valid on the server.

The Debian Locale guide itself is of course correct. And it states that you should not set LC_ALL=en_US.UTF-8 (!). That is however the only “working” answer I found online.

Solution
I guess the easiest thing is to add a line to your .profile file (on the server):

$ cat .profile 
.
.
.
LC_CTYPE="en_US.UTF-8"

If you are not using a bourne-shell you probably know how to set a variable for your shell. Any other method that will set LC_CTYPE to a locale valid on your Debian machine will also work. Clearly, you dont need (and you probably should not) set LC_ALL.

Not happy with en_US.UTF-8?
Not everyone use the American locale. To find out what locales are available/valid on your system:

$ locale -a
C
C.UTF-8
en_GB.utf8
en_US.utf8
POSIX
sv_SE.utf8

Want others?

$ sudo dpkg-reconfigure locales

or read the Debian Locale Guide.